The invention disclosed herein relates generally to flow sensing apparatus, and more particularly to a method and probe design for shielding a fragile flow sensor from contaminants in the flow being sensed while maintaining desirable performance characteristics.
Recent advancements in micro machining techniques have led to the development of solid state flow sensors of a type having a thin film heater and at least one thin film heat sensor suspended over a depression etched in a silicon substrate. Such sensors, referred to as microbridge flow sensors, are mass producible, very small and of potentially very low cost. In addition, such sensors are capable of very fast response. Reference may be made to U.S. Pat. No. 4,478,076 issued Oct. 23, 1984 in the name of P. J. Bohrer, and publications cited therein, for a detailed description of the fabrication techniques, structure and performance characteristics of microbridge sensors.
Basic sensors of the above-described type are sensitive to mass air flow, which is a parameter of considerable importance in a number of applications, including combustion control systems, and particularly including the control of fuel-air mixtures in internal combustion engines in which it is necessary to determine the mass air flow into a mixing or combustion chamber.
One of the problems encountered in utilizing microbridge mass air flow sensors in automotive applications is the presence of particulate matter, oil droplets and other contaminants in the air flow. Microbridge sensors are, in some respects, inherently quite fragile, and are subject to impact damage. Even through the combustion air is filtered to remove particulate matter and other contaminants, the air flow past the sensor may, nevertheless, have contaminants suspended therein. The air volume flow rate into an internal combustion engine is typically quite high, and may approach 1000 Kg/Hr. Contaminants carried in this air flow can destroy a microbridge sensor. Accordingly, precautions must be taken to prevent contaminants from impacting the sensor element.
Various techniques and probe designs have been devised in attempts to alleviate the foregoing problem. For example, the sensor element may be protected by its own dedicated filter, or the sensor element may be positioned or structure provided so that the element is located out of the main air stream.
These techniques and arrangements, though more or less effective in protecting the sensor element, introduce other problems. Filter type arrangements generally provide increased impedance to fluid flow, thus affecting a sensor's performance and generally decreasing its sensitivity to the total air mass passing the sensor location. Placing the sensor element out of the main air stream generally decreases its ability to sense the mass flow rate of interest.